I have seen the photos dated 1888. This may be based upon the professional photographer that took the photos and his time period working in the area.
When I stop and think about it this may make some sense -- or even more sense.
The RPO compartment did not go into the combines until the postal contract was obtained after the completion of the line into Owens Valley. I think the box car conversion dates from that time -- after the consturction period.
The loads on the flat cars appear to be like mine timbers and planks than like railroad ties.
Now, if it is during the construction period there is a fair chance that we may be able to determin a possible date. There are the dispatcher sheets for much of that time period in the CSRM collection. These "Train Register" sheets were used for 3 or 4 days each as there was usually one train a day and some extras with construction supplies. These give the locomotive number and the number of cars. During the construction period there were trains of 18 or more cars and at least one of 22 cars.
Brian Norden